Move input text update to function

Just a simple refactor.

Little bit more DRY and I needed to be able update the underlying start and end dates.

The problem was that the input didn't get updated to match that change.  I can force that update now by calling ```$(input).data('daterangepicker').updateInputText()```.

Please let me know what you think.  Also, vim stripped your whitespace, so sorry for the messy commit.